1. Field of the Invention
The present invention relates to a bushing for a drilling rig and more particularly to a bushing suitable for retaining a spider for holding a tubing string.
2. Description of the Related Art
In a conventional drilling rig structure, a length of tubing string, composed of individual tubing sections joined end to end, extends through an opening in the rig floor. A tubing string may extend for several thousands of meters below the rig and therefore may have a considerable weight. Complex devices have been designed for holding, lifting, and rotating tubing strings.
In a typical system, a so-called “rotary table” is mounted within the opening in the rig floor. In the case where a tubing string is being used to rotate a drill bit located at the bottom of the string, a “Kelly bushing” is fitted into a central region of the rotary table (the Kelly bushing may in turn be fitted within a “master bushing”). The Kelly bushing has a hexagonally shaped passage extending through a central region. The tubing string passes through the Kelly bushing and a hexagonal outer surface region of the tubing string is engaged by the bushing. The Kelly bushing can be rotated by the rotary table to in turn rotate the tubing string and the drill bit. Other types of bushing may alternatively be fixed into the rotary table to provide for different operations and/or tubing string configurations.
Is used. A spider is a complex piece of equipment and comprises one or more hydraulically driven grippers. In use, the spider sits on top of the rotary table, with a tubing string passing through the centre of the spider and being held in place there by the grippers. The grippers can be released to allow the controlled lowering and raising of the tubing string, e.g. using an elevator.
It will be appreciated that a spider is a very large and heavy piece of equipment, and may extend to a height several meters above the surface of the rig floor. It is often necessary for operators to work above the spider, for example to join a new piece of tubing section to the top of a tubing string. This requires the operator to stand on top of an elevated platform or ladder. It will be appreciated that this can present certain risks, especially where the operator is using heavy and awkward equipment such as a power tong which is used to join sections of tubing string together.
In an attempt to overcome this problem, it is known to provide a spider which can be fitted into a space in the rig floor so that the spider is suspended beneath the floor. For example, a spider has been designed which can be located within the rotary table (after removal if necessary of the Kelly bushing), with the spider having an integral flange at an upper end thereof, which locates into a recess surrounding the opening in the rotary table, to support the spider. This spider is not a standard conventional spider and rig operators must therefore discard there old spiders and purchase a new one. Furthermore, the new spider requires a new design of adapter plate/bushing which fits into the rotary table to support the spider.
The new design of spider is suitable for use in situations where the tubing string can be rotated by way of a top drive. A top drive is typically connected to the upper end region of a tubing string. However, this spider cannot be used in situations where it is required to rotate the tubing string using a conventional Kelly bushing mounted within a rotary table this requires the use of a conventional deck-mounted spider. In order to accommodate all possible scenarios therefore, it is necessary for rig operators to maintain both a conventional spider and a below deck spider in their inventories. It has already been noted that spiders are extremely large and heavy and the need to retain two spiders represents a considerable inconvenience. In addition, the requirement for two spiders increases a rig operator's costs.
It is an object of the present invention to overcome or at least mitigate the above noted disadvantages of spiders in general and of below floor mounted spiders in particular. It is a second object of the present invention to enable a single spider to be used both above and below floor level. It is a further object of the invention to enable a conventional spider to be located below floor level. These and other objects are achieved as least in part by providing a bushing which can be suspended through an opening in a rig floor and in which a substantially conventional spider can be located.